In November, Trump appeared to mock a New York Times reporter, Serge Kovaleski, who is physically disabled, though Trump denied he was doing so.

"Anybody [who] disparages people with disabilities, it sets me off," Bush is seen saying in the new ad. "That's why I called him a jerk."

It's not the first time Bush has forcefully attacked Trump, and the repeated attacks – along with their apparent failure to stick to "Teflon Trump" – reveal insights into the failures, and successes, of both candidates and their campaigns.

"What Jeb is desperately trying to do is find his swagger right now," GOP strategist Ford O'Connell told NPR. "The knock against Jeb is that he's low voltage and not willing to fight. The best way to shake those perceptions is to engage against the person who is in the media on a 24/7 loop."

Indeed, since his poll numbers dipped into the single digits late last summer, Bush has turned to negative campaigning in an effort to turn his campaign around.

He's alternately called Trump a "jerk," "a closet Democrat," and "unhinged," accusing the billionaire businessman of running a "chaos campaign." He's torn into Trump for his comments on deporting Latinos, barring Muslims, and mocking people with disabilities.

Of course, Trump himself has attacked Bush repeatedly, calling him "low energy," criticizing brother George W. Bush, and taunting his rival for his low poll numbers.

Now, Bush's Right to Rise USA super PAC, which had vowed to focus on positive advertising, is breaking course. Roughly two-thirds of its ads are now negative, and one-third biographical.

Why the uncharacteristic barrage of attacks from a candidate who once vowed to run a positive, joyful campaign?

"I think it's good politics for Jeb," Fergus Cullen, a former New Hampshire Republican Party chairman, told NPR. "There's 25 percent of the Republicans who are entertained by Donald Trump. But there's 60 percent of the party who say they won't vote for him under any circumstances. Those aren't Jeb Bush's people to begin with. He's trying to appeal to the other 60 percent by being the adult in the room and trying to govern."

Indeed, Bush's campaign certainly recognizes that Trump is not going anywhere and that Bush’s early failure to hit back makes him look weak.

"In a two-person race, negative ads can be effective. But in a multi-candidate race, negative ads don't usually help the person who uses them," says Jim Broussard, professor of history at Lebanon Valley College in Annville, Penn. "The ads might drive up the net disapproval percentage of the person they are aimed at, but they also usually drive up the disapproval rating of the person who issues the ad as well."

That's because there's a chance voters will associate Bush's vitriol with Bush himself, rather than with Trump, according to David Hagenbuch, a professor of marketing at Messiah College.

"Even if we’re speaking about someone or something else, our choice of words and our tone paint a picture of who we are," says Prof. Hagenbuch. "For instance, if we use vulgarity or profanity to describe what another person has said or done, there’s a good chance our hearers will remember us as being vulgar or profane as much as they’ll remember what we’ve suggested about the other person.”

In other words, when Bush calls Trump a jerk, there's a risk voters will associate that word with Bush as well.

Trump, meanwhile, who's ratcheted up the barrage of insults against Bush, has gained – he's at 28 percent.

Why can't anyone scratch "Teflon Trump"?

Many of the candidates who have tried – like Sen. Lindsey Graham, Gov. Scott Walker, Gov. Bobby Jindal, and Gov. Rick Perry – faced blistering attacks in response and were eventually forced to drop out.

"Trump's supporters have a strong emotional bond with him. They see him, billionaire and all, as 'one of them,' like a member of their family," he says. "So when any other candidate – or the media – criticize Trump, his supporters think 'they are attacking me!' and that only reinforces their support for him."

"Imagine that someone attacked a member of your family. You would be far more likely to rally to his support and denounce the attacker than you would be to believe the attack. So the worst thing the other candidates can do is to attack Trump; it only binds his supporters closer to him."